Document WO 2004/063560 A1 discloses how a fuel/air mixture can be ignited in a combustion chamber of an internal combustion engine by a corona discharge created in the combustion chamber. For this purpose an ignition electrode is guided through one of the walls, that are at ground potential, of the combustion chamber in an electrically insulated manner and extends into the combustion chamber, preferably opposite a reciprocating piston provided in the combustion chamber. The ignition electrode constitutes a capacitance in cooperation with the walls of the combustion chamber that are at ground potential and function as counterelectrode. The combustion chamber and the contents thereof act as a dielectric. Air or a fuel/air mixture or exhaust gas is located therein, depending on which stroke the piston is engaged in.
The capacitance is a component of an electric oscillating circuit which is excited using a high-frequency voltage which is created, for example, using a transformer having a center tap. The transformer interacts with a switching device which applies a specifiable DC voltage to the two primary windings, in alternation, of the transformer connected by the center tap. The secondary winding of the transformer supplies a series oscillating circuit comprising the capacitance formed by the ignition electrode and the walls of the combustion chamber. The frequency of the alternating voltage which excites the oscillating circuit and is delivered by the transformer is controlled such that it is as close as possible to the resonance frequency of the oscillating circuit. The result is a voltage step-up between the ignition electrode and the walls of the combustion chamber in which the ignition electrode is disposed. The resonance frequency is typically between 30 kilohertz and 3 megahertz, and the alternating voltage reaches values at the ignition electrode of 50 kV to 500 kV, for example.
A high-frequency corona discharge can therefore be created in the combustion chamber. The corona discharge should not break down into an arc discharge or a spark discharge. Measures are therefore implemented to ensure that the voltage between the ignition electrode and the combustion chamber walls, which are at ground potential, remains below the voltage required for a complete breakdown.
The space that is available in an internal combustion engine for guiding the ignition electrode, and the insulator enclosing same, through a combustion chamber wall, in particular through the cylinder head of a piston engine, is limited, especially in modern engines for passenger vehicles, in which case a threaded hole of M10 to maximum M14 is typically provided for screwing in a spark plug, and therefore an outer diameter of no more than 10 mm is available for the insulator of an igniter according to the invention. Moreover, there are demands to further reduce the size of the threaded holes in the cylinder head. Considering the high requirements placed primarily on the insulation capacity of the insulator—high voltages in the range of 50 kV to 100 kV at frequencies in the range of 30 kHz to 3 MHz, combined with small passage openings in the combustion chamber walls, high and fluctuating pressures and temperatures in the combustion chamber, and attacks by the combustion chamber atmosphere—engineers involved in the development of a igniter according to the invention for internal combustion engines face considerable challenges, especially since the continual reduction in diameter of the outer conductor and of the ignition electrode in particular increases the risk that the insulator will become overloaded by the high voltages and electric field strengths. Given the high voltage between the ignition electrode and the surrounding outer conductor required to generate a corona discharge, as the outer diameter of the ignition electrode is continually reduced and the inner diameter of the outer conductor is continually reduced, so does the risk increase that the maximum electric field strength generated by the high frequency will exceed the breakdown strength of the insulator and voltage breakdowns will occur in the insulator.